"Sexist" data is making it harder to improve women and girls' lives, the world's leading philanthropic couple Bill and Melinda Gates have said in an open letter.

The couple warned that a lack of focus by researchers on gender and a disdain for what were perceived as "women's issues" were resulting in "missing data" that could lead to better decisions and policies, enable advocacy and measure progress.

"The data we do have - data that policymakers depend on - is bad. You might even call it sexist," Melinda Gates wrote in their annual letter discussing the work of their foundation, one of the largest private charities in the world.

Gender inequality is one of the greatest barriers to human progress, the United Nations said last year, with studies showing that when girls stay in education, they have more opportunities and healthier children, which boosts development.

But data often does not take gender into account and is flawed by biased questions, said the husband and wife team behind the Bill & Melinda Gates Foundation.

Because women in developing countries are primarily seen as wives and mothers, most of the data about them focuses on their reproductive health, not their earnings and assets, they said.

"You can't improve things if you don't know what's going on with half the population," wrote Gates, co-founder of Microsoft Corp.

The couple said mobile phones offered a powerful tool to allow women to build new connections, gain economic freedom and challenge restrictive social norms, for example by buying contraceptives online.